The present invention relates to a compact quick-cooking convection oven and more particularly to such an oven which is suitable for residential use that is, use in a home as opposed to a commercial establishment.
Compact quick-cooking convection ovens designed primarily for use in commercial establishments are described in U.S. Pat. Nos. 5,254,823; 5,434,390; 5,558,793; and 5,927,265 (hereinafter xe2x80x9cthe cited prior art ovensxe2x80x9d), which patents are incorporated herein by reference. Such ovens have proven themselves to be satisfactory in use in a variety of commercial establishments. However, they are not uniquely adapted to meet the requirements for residential use where considerations of available voltage, size, warm-up time, operating and purchase costs, and the like may be quite different.
For example, an oven according to the cited patents, for use in a commercial establishment, may have cooking chamber dimensions of 5xe2x80x3 high by 18xe2x80x3 deepxc3x9718xe2x80x3 wide. By way of contrast, an oven suitable for residential use should preferably have cooking chamber dimensions about 12xe2x80x3 highxc3x9714xe2x80x3 deepxc3x9724xe2x80x3 wide. As another example, an oven situated in a commercial establishment will generally have available to it three phase 220-240 voltage. By way of contrast, in a residence in the U.S.A. the available power for an oven will generally be single phase 220-240 voltage. As a further example, an oven for use in a commercial establishment may have a substantial warm-up period (for example, thirty to sixty minutes) since the oven is only turned on once during the work day (typically well before any customers are allowed to enter the establishment) and then kept on throughout the work day. By way of contrast, an oven suitable for use in a residence should have a very brief warm-up time (typically less than 10 minutes), since it will probably be turned off between meals (that is, about three times a day), with a warm-up time being required after each turn-on. For the above-mentioned reasons and numerous others, a residential oven must adjust or compromise various features in order to achieve the same highly desirable rapid cook time as the commercial oven of the cited patents.
The cited prior art ovens feature opposed primary energy flows with flows of hot gas striking the upper surface of a food product within the cooking chamber and with microwave energy being launched upwardly from the floor of the oven into the lower surface of the food product. To provide bottom side convection heat transfer, the hot gas flow is pulled around the sides of the food product (from the upper surface thereof) and across the bottom of the surface of the food product by a low pressure gas return passage located directly below the food product. This produces a hot gas xe2x80x9cshroud effectxe2x80x9d about the food product. The desired hot gas flow beneath the lower surface of the food product is accomplished by using a food product-supporting, microwave-transparent, ceramic cooking platter which forces the gas flow along the lower surface of the food product (the food product being supported above the cooking platter by standoffs) before the gas flow can pass downwardly through the hole(s) of the cooking platter and exit downwardly from the cooking chamber.
The microwave energy is launched from below and enters the food product only after passing through the microwave-transparent cooking platter.
To generate the desired bottom launched microwave feed into the cooking chamber, a microwave launch cavity and wave guide feed are located below the cook chamber. The launch cavity is roughly 9 inches in diameter and 5 plus (1xc2xd wavelengths desired) inches high, with a mode stirrer located near the top of the cavity. This cavity typically projects about 1 inch into the cooking chamber cavity. It is isolated from the cooking process by an environmental seal which is a 9 inch diameter microwave-transparent window (high temp) and a grease/water seal to prevent water from entering into the microwave launch area. This resonant cavity couples the microwave energy primarily directly to the food, with secondary cavity coupling. As a result, the volume associated with the microwave launching/feed kit is large and has the effect of limiting the overall product packaging (e.g., oven size) and configuration (reduced cooking chamber size given the launch cavity volume). It also negatively impacts the cooking chamber design because it limits cleaning of the chamber bottom. Given the large diameter can, the microwave feed, and the plug seal, the current microwave kit is complex and expensive to manufacture. It also requires a mechanically/motor driven mode stirrer (motor/gearbox, shaft, microwave seal, and stirrer blade) located in the launch cavity. In addition, the construction of the launch cavity, its xc2xc wavelength matching feed section, and the plug seal (microwave transparent window and environmental seal) is expensive.
In the cited prior art ovens the platter channeled the hot gas flow below the food and had three primary functional requirements: (1) to support the food, (2) to stand the food off the platter upper surface and thereby create an gas flow path between the platter and the food, and (3) to be microwave transparent so that the electromagnetic microwave energy launched from below the food can pass through the food support (platter). These requirements lead to the use of a cast ceramic platter which has the desired microwave transparency properties and can be formed with a number of standoffs used to create flow channel(s) defined by the platter upper surface and the bottom surface of a food item or cooking dish. In addition, the ceramic plate is cast with several holes which permit the gas flow to exit the flow channels to the blower return. This sophisticated cast ceramic part is expensive, fragile, and difficult to clean.
Further, the platter is complicated and difficult to produce. For the platter to provide adequate heat transfer to the food, a substantial portion of the gas flow must be channeled between the food and platter. To achieve this, the platter must have a tight fit (small clearance) to the oven walls in order to prevent the gas flow from by-passing around the platter and flowing directly to the gas return passage. Minimizing flow by-pass between the platter and the door, coupled with the door features for controlling microwave leakage, has resulted in the oven door covering the cook zone, such cook zone being defined by the platter at the bottom, the cavity roof at the top, and the oven walls therebetween. In essence, a two cavity oven results: an upper chamber containing the cook zone, and a lower zone below the upper chamber containing the gas return space. Such a two cavity construction is more expensive to produce, given the presence of the lower chamber or return gas volume which is not required for standard ovens. This lower chamber also results in additional cleaning difficulties for the user or consumer as the platter must be removed and the lower chamber cleaned of food or grease that may spill passing through the holes in the platter, or be deposited by the gas carrying grease/food particles flowing through space below the upper chamber.
In the cited prior art ovens, both the convection heating subsystem and the microwave heating subsystem are electrically powered, with the majority of the power expended being allocated to the convection heating elements. The primary energy flows are as follows:
1. Convection Top: Hot gas flow heat transfer onto the upper surface of the food;
2. Microwave Top: Microwave energy input that passes through the cooking platter, but xe2x80x9cmissesxe2x80x9d the food, reflects off the upper surfaces of the cooking chamber, and becomes absorbed by the food through the food upper or side surfaces;
3. Convection Bottom: Convection heat flow across the lower surface of the food; and
4. Microwave Bottom: Microwave energy input from the bottom of the cooking chamber, through the platter, through the bottom surface of the food product, and into the center of the food product (primary microwave input).
In the cited prior art ovens, when the energy (power) flows of the convection gas and microwave energy are at full capacity, the total energy flow into the upper surface of the food product is about 1,900 watts (1,000-1,300 watts top convection and 400-600 watts top microwave) and the total energy (power) into the lower surface of the food product is also about 1,900 watts (500-700 watts bottom convection and 1,100-1,300 watts bottom microwave). Thus the energy split is roughly 2:1 for the convection energy, in favor of the top, and roughly 1:2 of the microwave energy in favor of the bottom. Actual distribution of the energy is a function of various factors including the geometry of the food, the geometry of the oven, etc. By having nearly the same quantity of energy delivered to both the top and bottom surfaces of the food, a uniform cook is obtained because the temperature profile is symmetric about a horizontal centerlinexe2x80x94that is, isotherms are established in the food. This energy split in microwave and convection energy between the top and bottom food surfaces is critical to obtaining a finished food product which is both rapidly cooked and of high quality. The energy split minimizes the use (or need!) of internal heat conduction within the food being cooked.
The several power inputs identified above must be tailored in order to produce cooking which is both high speed and high quality. Thus, most foods are cooked on a dish or pan which retards moisture loss from the bottom surface of the food. Further, the bottom surface of the food typically requires only a modest level of browning relative to the browning level required at the upper surface of the food. By way of contrast, the upper surface of the food undergoes more significant moisture loss, and typically the food product is cooked with a greater level of browning.
Accordingly, the majority of the microwave energy is introduced into and through the bottom surface of the food, while less than half of the convection energy is applied to the bottom surface of the food. On the other hand, the majority of the convection energy is introduced into the upper surface of the food to provide moisture loss therefrom (through evaporative cooling of the upper surface) and browning thereof, while less than half of the microwave energy is applied to the top upper surface of the food in order to prevent excessive heating thereof. Thus the energy ratio of convection energy to microwave energy is roughly reversed depending on whether one considers the top surface or the bottom surface of the food.
More particularly, in order to generate the high level of heat transfer desirable at the upper surface or top of the food, impingement-style (that is, forced hot gas stream) heat transfer is used in order to dissipate the relatively cool stagnant gas layer directly above the food. To generate the desirably high heat transfer (as high as 35 BTU/hr/ft2/xe2x80x3F), strong flows of the impingement gas must be used. To generate such strong flows easily and economically, a high velocity gas flow in combination with a modest gas pressure is used. This requires a blower which generates, for example, at least one horsepower at maximum operating conditions.
The cited prior art ovens present manufacturing and operational problems arising out of the xe2x80x9cshroud effectxe2x80x9d wherein hot gas flow launched from above is continually drawn down and around the food product so that it exits through the center of the cooking platter and thereby from the cooking chamber bottom, while microwave energy is launched upwardly from below the center of the cooking chamber bottom. For example, due to the co-location of the microwave feed into the cooking chamber bottom and the hot gas flow return path through the oven bottom to the blower, the oven bottom necessarily has a complex and expensive lower gas duct system. This follows from the fact that the gas that is drawn around the food product and exits the bottom of the cooking chamber is gathered up in an annulus with the center of the annulus being occupied by the microwave launching window.
Additional disadvantages arise from the point of view of the user. Thus the use of a single hot gasflow to provide both top and bottom surface convection heat transfer seriously limits the flexibility of food preparation as the top and bottom surface convection heat transfers cannot be independently controlledxe2x80x94for example, to permit additional bottom surface browning while reducing top surface browning. A further problem from the point of view of the user is that the central bottom of the cooking chamber is characterized by a region that is difficult to clean because access is limited. Spills, overflows and droppings from the food being cooked combine with grease carried by the gas flow to pass below the cooking platter and out of the cooking chamber into this access-limited and difficult-to-clean region.
The cited prior art ovens are thus subject to limitations and deficiencies in terms of manufacturing complexity and cost (due to the co-location of the central microwave feed and the gas return path to the blower), the difficulty of cleaning overflows and deposited grease (due to limited access), and the loss of cooking flexibility and control (due to the use of a single gas stream to provide both top and bottom surface convection heat transfer). Of special import to a compact oven suitable for residential use (and therefore subject to severe height constraints) is that a significant fraction of the cited prior art oven""s vertical space allocation must be reserved for the lower gas return means, at the expense of the vertical space available to the cooking chamber interior.
Accordingly, it is an object of the present invention to provide a compact quick-cooking convection oven suitable for residential use.
It has now been found that the above objects are obtained in a residential version of a compact quick-cooking convection oven for cooking a food product at least partially by hot gas flow. For residential use primarily, the compact quick-cooking convection oven for cooking a food product at least partially by hot gas flow, comprises a housing defining (i) a cooking chamber having at top, a bottom and a support means therebetween and spaced above the chamber bottom for receiving and supporting a food product for cooking, and (ii) conduit means for providing gaseous communication outside of the cooking chamber upwardly from the chamber bottom to the chamber top. Associated with the cooking chamber are (i) adjacent the chamber top, directing means for directing gas flow from the conduit means downwardly onto a top of the food product on the support means, and (ii) adjacent the chamber bottom, return means for directing the gas flow from the cooking chamber into the conduit means. Flow means cause gas flow from the directing means into the return means and from the return means into the directing means via the conduit means. Control means independently vary without human intervention at least one of the effective volumetric flow rate of the gas flow into the cooking chamber and the temperature of the gas flow into the cooking chamber. The cooking means are convection heating and additional heating selected from the group consisting of microwave radiant heating (electromagnetic energy), non-microwave radiant heating (infrared heating), and combinations thereof.
The convection heating includes at least one convection heating element selected from the group consisting of (i) a convection heating element disposed in the return means, (ii) a gas combustion burner disposed below the food support, and (iii) combinations thereof.
The microwave electromagnetic energy (radiant) heating, when present, includes at least one microwave energy source for the cook chamber selected from the group consisting of (i) a microwave launcher disposed beneath the food support, (ii) a microwave launcher disposed in the top of the cooking chamber; (iii) a slotted microwave launcher disposed in opposed bottom sides of the cooking chamber, and (iv) combinations thereof.
The non-microwave radiant heating, when present, includes at least one non-microwave radiant heating element selected from the group consisting of: (i) an upper non-microwave radiant heating element disposed adjacent and below the directing means, (ii) a lower non-microwave radiant heating element disposed in the return means and distributed along the area below the food support such that at least a portion of the gas entering the return means is initially reheated by one non-microwave radiant heating element prior to flow under the remainder of the food support, and (iii) combinations thereof.
In a preferred embodiment, the oven additionally includes at least one catalytic element selected from the group consisting of (i) a catalytic element disposed in the return means, (ii) a catalytic element disposed in the conduit means, and (iii) combinations thereof.